As I mentioned in the previous blog, there are several considerations when picking a pond pump for a fountain or waterfall.
First consideration is how much water you want to move. Pond pumps are graded by the number of gallons per hour they move, and most are marked with this on a sliding scale. As a general rule, you want to move all water in your pond through the pump every hour, or two at the most. My pond is about four foot in diameter and a foot and a half deep-- 4x4x1.5=24 square feet times 7.5, which is the number of gallons per square foot of water=180 gallons. Now my pond actually has sloping sides, so it'll be a little less than this, but we're still looking somewhere near 150 gallons, which means my pump needs to move at least 75 gallons per hour, or GPH.
Also on the side of my pump package was a note about how high it could lift that many gallons per hour. The amount of lift is generally referred to as head height. Every vertical foot you raise the water from where the pump is placed, to where the water comes out, is one foot of head height. My pond is 18" deep, and the water empties from the pump hose about six inches above the surface of the water, so I have two foot of lift--but since I'm also using the pump for circulation purposes, I have it placed on the far end of the pond, and then it curves around the back of the water fall, so the hose is nearly ten foot long. For every ten foot of pipe or hose, you must add another foot of head height. So the head height my pump has to handle is three.
In my next pond build, I plan to build a shallow pool at the bottom of my water fall, then a short stream, and finally the water will drop into the main pond where the fish will be. That means the hose/pipe could be nearly thirty feet from one end to the other, and will probably lift the water at least four feet, so my pump will need to lift the amount of water I want to move at a head height of at least seven feet.
The other consideration for the amount of water you want to move has to do with the width of a waterfall or stream that you plan to use, and how much you want to put through it. If you want a trickle of water, or about 1/8 of an inch across the width of your waterfall, you'll need to move at least 100 GHP for each four inches of spout. For a somewhat more impressive display of 1/4 inch of water, you'll need 100 GHP for every 2 inches of spout, or for a real thick waterfall of 1/2 inch of water, you'll need 100 GHP for every inch of spout or falls. The numbers are similar for a stream--if you want a six inch stream with water running 3/4 of an inch deep (and remember you'll have gravel on the bottom, so you'll need at least that much water so you can actually see it) you'll need a pump that will move at least 900 GHP of water.
In my little pond, my pump will move about 150 GHP at the three foot of head height. It isn't an impressive water fall display, but it keeps things circulating, and provides a nice trickling sound.
Some people leave their pumps off unless they are out by the pond, but I prefer to have mine running all the time for optimum pond health.
A third consideration, especially if you are going to lave a larger pond, is using a filter, and how much power you'll need to push the water through any filter materials. For filtration, the pump should move the entire volume of the pond every hour, at at the very least, every two hours. Erring on the side of caution is best, and different filters require different amounts of pressure--and there are other options for filtration available as well, but that is fodder for another blog.
One last note on pumps: there are two main kinds: submersible, and non-submersible.
A submersible pump goes directly into the pond. This is ideal for smaller ponds, or if you're trying to keep a hole open in the ice. A larger pump for a big pond or water feature might be better off being located outside the pond. These pumps are a little more money, but there are lots of benefits of using this method.
As a general rule, you get what you pay for with pumps. I know they can be pricey, but I wouldn't skimp if you can help it since a cheaper pump is more likely to have problems down the road. This isn't always true, and you may be able to shop around and find a great deal of a high-quality pump. Sites like Craig's list--or in Utah, KSL.com has a great classified section--could be a real benefit if you are careful and have time to watch for a good deal.